Steven R. Green

Transpiration and root water uptake by olive trees

While the cultivated olive tree (Olea europaea L.) is known to be sclerophyllous and effective at tolerating drought, little is known of its short-term water-use dynamics for most studies have been based on longer-term, water-balance information. We present here, for the first time, heat-pulse measurements of the sap flux measured not only within the semi-trunk of an olive tree, but also within a root excavated close to the stump. One tree in the olive grove near Seville in Spain had regularly received basin irrigation during the summer, whereas the other, growing on this deep silt loam, had been without water for over 3 months. Following a flood irrigation of 730 L to a dyked area around the tree, the regularly-irrigated olive maintained a transpiration rate of 1.65 mm3 mm−2 d−1, on a leaf area basis, for only 3 days following the irrigation. This rate was maintained for a total consumption of 110 L. It then began again to limit its rate of water use with transpiration falling below that predicted for well-watered conditions by the Penman-Monteith equation. The flow of sap in the near-surface root dropped concomitantly. Meanwhile the unirrigated tree was using water at just 0.78 mm d−1. Yet following an irrigation of 870 L it only lifted its consumption to 1.12 mm d−1, on a leaf area basis. Neither did it recover its leaf water potential following this wetting because of an inability to refill cavitated vessels. These data again show olive to be a parsimonious and cautious consumer of soil water.

Rootzone processes and the efficient use of irrigation water

Abstract The need for more-efficient agricultural use of irrigation water arises out of increased competition for water resources, and the greater pressure on irrigation practices to be environmentally friendly. In this review for the 25th Jubilee volume of Agricultural Water Management we focus on three rootzone processes that determine water-use efficiency in irrigation. Firstly, we discuss the role of macropores in preferentially-transporting irrigation water to depth during infiltration under both sprinkler and flood systems. It is suggested that more-uniform entry of irrigation water into the rootzone will result either by matching the sprinkler rate to the soils matrix hydraulic conductivity, or by modifying the soil-surfaces macroporosity prior to flood irrigation. Secondly, the environmentally-deleterious leaching of chemicals by irrigation is shown to be reduced if the applied fertilizer is first washed into dry soil by a small amount of water. This first pulse of water is drawn by capillarity into the soils microporosity, and it carries with it the dissolved fertilizer which becomes resident there. These nutrients are then available for plant uptake, yet less prone to susbsequent leaching by heavy rains. Meanwhile, initially-resident solutes in the dry soil, such as salts, will be more-effectively displaced by the infiltrating irrigation water. Finally, our time domain reflectometry (TDR) observations of the changing soil water content in the rootzone of a kiwifruit vine, and our direct measurements of sap flow within individual roots, both reveal that plants can rapidly change their spatial pattern of water uptake in response to the application of irrigation water. The prime uptake role of near-surface roots is highlighted. Consideration of all three of these rootzone processes reinforces the claim that more- efficient and environmentally-sustainable water management will arise through higher-frequency applications of smaller amounts of irrigation. Key words: Rootzone process; Water-use efficiency; Macropore: Sprinkler and flood systems *Corresponding author. 0378-3774/94/

Prediction of Groundwater Nitrate Contamination after Closure of an Unlined Sheep Feedlot

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Anion Transport Involving Competitive Adsorption during Transient Water Flow in an Andisol

Nitrate contamination of groundwater by a sheep feedlot in Hawke9s Bay, New Zealand led to closure of the feedlot in 1998. However, knowledge of the processes controlling how long the contamination will remain and an analysis of whether the current land use (vineyard) will also impact groundwater quality are required to assess long-term groundwater quality issues after feedlot operations cease. To determine the fate of NO 3 following land use changes and the rate of reduction of contamination that may be expected under natural conditions following such changes, we compared the chemical concentrations of NO 3 –N, Cl, and alkalinity (HCO 3 ) in the groundwater and rivers from surveys conducted in 1994 and 1995 with sampling conducted in 2001. Profile sampling of total N and C of the 3 –N concentrations were under the feedlot (>140 g m −3 NO 3 –N) and down gradient. In 2001, 3 yr after the feedlot closed, the Cl concentrations had increased in down-gradient wells but remained similar to the 1994 survey in other wells. There has been a decrease in NO 3 –N concentrations in most wells, compared with the peak NO 3 –N concentrations recorded in the 1995 survey, but an increase compared with 1994. Alkalinity concentrations in wells located within the influence of the feedlot are approximately 150 g m −3 lower than in surrounding wells. This indicates that nitrification reactions are affecting the HCO 3 concentrations in the feedlot-influenced wells. However, the HCO 3 concentrations of some of these wells are increasing, indicating that nitrification could be slowing down and the aquifer is beginning to recover. SPASMO modeling indicates that NO 3 contamination from the site will continue for the next 3 to 5 yr. The impact of NO 3 leaching due to current land use practices is likely to be much less than the feedlot. The model predicts there will be an improvement in groundwater quality in the next 3 to 5 yr as NO 3 from the feedlot eventually leaves the vadose zone profile and mixes into the unconfined aquifer.